Transmission control in signaling systems



April 26, 1938. A. c. DICKIESON TRANSMISSION CONTROL IN SIGNALING SYSTEMS Filed 001;. 29, 1936 y N 5 w M III' R m E m mobu q N K T 55 E C A V l w W I m A mi 5%. J fifi y 7 a 553% B 20k u Qiouuu Patented Apr. 26 1938 UNITED STATES,

TRANSMISSION CONTROL IN SIGNALING SYSTEMS Alton C. Dickieson, Hollis, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York 1 Application October .29, 1936, Serial No. 108,190

2 Claims.

The invention relates to signal wave transmis sion systems and particularly to wave-operated switching circuits for controlling signal transmission in such systems.

5 The invention has particular'reference to carrier wave signaling systems of the type in which carrier energy, or special control energy other than the carrier, is applied to the transmitter only during periods of signal transmission, and

in which the gain of the receiver is automatically controlled by the received carrier or other control energy.

In systems of the above type, every time the carrier or control energy is applied, the receiver gain changes from a maximum value as limited by the manual control to a value determined by the carrier amplitude. To allow for correction for fading, the range of gain needs to be at least 10 decibels, and may be as much as 50'to 60 decibels. That is, every time the carrier is applied during the gain adjusting intervaL'the gain is considerably higher than it should be finally, with consequent high noise and speech distortion. For control current operation where nearly every word means a new application of carrier, these transient effects will be quite troublesome.

So-called "Codan (carrier-operated detector anti-noise) circuits have heretofore been used with the receiving circuits in such systems, to

prevent the reception of interference (static) which, during periods of no signal (no carrier) when automatic gain control receivers are at maximum sensitivity, may be of sufiicient ampli- J, tude to annoy the receiving subscriber and to falsely operate the voiceeoperatedswitching circuits for controlling the application of carrier by the transmitter at the terminal. The codan circuit accomplishes this by disabling the output 40 of the receiver at each stationduring periods of no signal transmission and removing'the disability when signal transmission is taking place."

The fundamental requirements of a codan are 1) that it operates on the weakest carrier that will give a commercial circuit with the static and set noise present and (2) that it should not operate on the strongest static through which a commercial circuit could be given with the circuit elements involved. The attaining of these 50 requirements is made d'ifiicult in presentday radio receivers because the codan is controlled from a point in the receiving circuit where the signalto-noise ratio (noise considered with carrier ofi) is made to approach unity, no matter how large that ratio may be at the receiving antennafdue to the action of the preceding automatic gain control; This diflicultyhas been minimized inthe past 'by the use of a manual gain control to adda loss inside the circuit, the gain of which "is being adjusted, to limitthe maximum gain of the receiver, and thus effec tively to preventthe automatic gain control from operating'in the region Where the static falls. This method is not particularly suitable at an attended station because of the wide variations in conditions requiring qui-ck adjustments in gain, and, of course, is not feasible for an unattended station. a r

An object of the present invention is to reduce in systems of the above type signal distortion at the receiver due to the removal and reapplication of the carrieror other control energy. Another object is 'to pro'vide-a receiver'gain of an optimum value when the carrier or control energy is removed and to quickly readjust the gain tothe proper value when carrier or control energy is again applied, if a re-adjustment is necessary. I

These objects are attained in accordance with the invention by so arranging the codan in the receiving circuit that it is controlled by the received carrieror control energy prior to its regulation in level by the automatic gain control and to make the gain regulation neutral, that is, to

maintain the receiver gain at the last established value when the reception of carrier or control energy ceases until the carrier is again received and then to quickly readjust the gain to the optimum value for signal reception.

Other objects and features of the invention will be brought out in the following detailed descriptionwhen read in connection with theaccompanying drawing, the single figure of which shows diagrammatically (and in part schematically). a two-way radio telephonesystem em-- I bodying the invention. 1

The westterminal of the system in'the drawing is shown diagrammatically. This portion is. not an actual circuit diagram butrather a single line layout, each line indicating a two-wire transmission path. A break in a path, represented by separated arrowheads, indicates that the path is normally disabled at that point,'and an arrow pointing toward such a break from a control device (amplifier-detector) indicates that the path will be made operative by operation of the control device. A make in the path, represented by contacting arrowheads, indicates that the path is operative at that point, and an arrow pointed towards the contacting arrowheads from a control device (amplifier-detector) indicates that the path will be disabled at that point by r operation of the control device. An arrow'pointing from one box representing acontrol device (automatic gain'control or codan) 'to' another box representing other apparatus indicates that the former device controls in some manner the V v a 'paratus identical with that at the west terminal ,is shown by a circuit diagram'in which thecomadjustment or rearrangement of the latter appa-f ratus, a "i V f The cast terminal of the system employing ap- *ponentparts of the apparatusiand circuits par:

- The system of ticularly pertainingto the'invention are: shown in greater detail.

the drawing is a two-way radio telephone system for transmitting telephone sighalsin opposite directions betweena "west two waytelephone line LW' and an east two-way telephoneline v 7 a r r .jThe west terminal of the radio telephone systemicomprises the transmitting circuit ineluding a transmitting'iantenna TAW, and a receiving circuit RW including a receiving antenna RAW', the input, of the circuit 'I'W and the output of the circuit RW being coupled in conriusa r elation "witheach other; and in energy I transmitting relation with the telephone line LW by the hybrid coil connection Hrandgbalancing network N1 in well-known manner.

1 Similarly, the eastterminal ofthe radio tele-' phone system comprises a transmitting "circuit 'I'Eincluding the transmitting antenna TAB and the receivinggcircuit RE including the receiving antenna RAE, theinputrof' the circuit TE and the output of the circuit REbeing coupled inconing'network N2. '7

' ciated carrier oscillator 2. In the'absence of speech wave transmission therein, the circuit 'TW is arranged in any'suitablemanner, asiin- V dicated byxthe' normal open circuit 33 between transmitter I and transmitting antenna TAW,

to prevent the radiationof carrier oscillations to the east terminal station. When speech wave signals are received in the transmitting circuit TW from the line LW, a portion thereof is im- 1 pressed on the input of the wave-operated 'control device (amplifier-detector) Lbridged'across the circuit TW in front of the radio transmitter I, and causes theoperation of that device to remove'th e disability in the circuit TW atthe point 3 thereby allowing one or'both ofthe' signal side bands, as well'as theunmodulatedjcarrier component, to be radiated by the transmitting an= tenna TAW to theeast terminal of the system.

, Thereceivingicircuit -RE in "theveast' terminal of 'thesystem comprises the usual? transmission o apparatus "including the radio frequency tuning] 65". Y 7

' associated carrier'oscillator I, the intermediate and amplifier means-5, affirst detector 6 and frequency amplifier 8,'the second detector 9 and the audio amplifier I0.

Associated with the receiving circuit RE of the east terminal isthe codan circuit I I. As indicated, itcomprises the amplifier-detector cir- V cuit I2, "selective to a beat frequency, i."e., the

difference betweenthe two carrier frequencies f1 and f2, respectively generated by the carrier oscillator 2" at the 'west terminal'and the carrier "by; operation of relay I3.

oscillator l at the east terminal, having its in-: 7 put connected across the receiving circuit RE at a point between the first detector 6 and the intermediate frequency amplifier 9, .the mechanical relay I3 adapted to be operated by 'detected-current inthe output of the amplifier-detector IZ,Q

and the normally closed short circuiting connec- 1 tion I5 across the'receiving circuit RE; in the out i Ill adapted to beopened 7 10 Also, connected across thereceiving circuit RE atthe east terminal ata point between the out-i put'o'f the audio'amplifierf put of the intermediate frequency amplifier 8 and the input of the second detector 9 is the input of the backwardacting portion '14 of an auto- 1 matic gaincontrol' circuit for the intermediate amplifier '8', comprising the three-electrode vacusum tubezdetector lfi having its'input connected 7 'across the receiving circuit RE by inputtransformer IT a gain decrease relay GD and a gainincrease disablerf relay GID controlled'zby tector 5;. and associated the detected carriercurrentin the'outputof delcircuits controlled: by; 1

'operation'of these relays. -Theautomatic gain control circuit also co-r'nprises 1a "forward-acting portion including the amplifier-detector I Z common also to the codan'circuit II, the-gain ins"; f

crease relay GI controlled by the detected car-V rier current, and an associated circuit controlled by operation ofrelay GI. automatic gain control'clrcuit is provided for regulating the gaina of the receiving circuit'RE 'to compensate'for-j variations in'received signal volume, in a'rnan ner which will'be describedin detaillater in con: nection with the description of operation of the: complete system;

The transmitting circuit iIE at the east termi: nal of the system contains transmission and con trol apparatus similar to that used in the trans mittingcircuit 'IW atthe west terminal of-the system, as'indicated by the useoi-si'mil'ar charact ters for identifying corresponding pieces or ap-f paratus; eXcept-thatin the circuit TE the char' acters are followed by a prime 1) mark: The frequencies of the carrier oscillators, 2, Z in the transmitting circuits at the' two terminals'are indicated as being the same, f1, but may be different irequencies. Also, the'receiving circuit RW at the west terminal of the system contains transmission and control apparatus similar to that used in the receiving circuit RE at the east terminal including a codan circuit and an auto maticfgain control circuit as described above as identifying corresponding fpieces ofapparatus,

followed by afp'rime mark.

' Forthe purpose of giving exclusive controlof' the switching apparatus atfboth'terminals to the particular subscriber associated 'with' the "tele-' indicated byjthe use of {the same characters for" except that in the circuit RW' the characters are 7 j phone line LW or LE, who first istartsito talkj detector) 4 at the west terminal in responsejto telephonesignals in the circuit 'IW, in addition to rendering the latter circuit operative at the point 3 to transmit signal side bands and un modulated carrier component to thetransmitting antenna to be radiated; also disables the" normally operativeoutput of the codan circuit additional switching apparatus'i r -a i both terminals as follows: v Q 1 Operation of the control circuit (amplifier? II and a portion (gain increase) of the auto 'matic gain control circuit I4 at the points-I8;

as indicated. Similarly, operation of the control circuit .4 at the east terminal in response to with the east subscriber.

. known manner.

telephone signals in the transmitting circuit TE, disables the codan circuit H in its output and the gain increase action of the automatic gaincontrol circuit 14, for example, by causing the operation of a relay I9 to break the connection betweenthe output of the common amplifier-detector l2 and the codan relay l3 and the gain increase relay GI at the point 20, as indicated.

Also, operation of the amplifier-detector l2 at the east terminal, if its output is operative, causes the operation of the control relay l3 to disable the amplifier-detector 4' in its input, for example, by closing an energizing circuit for a relay 2| to cause the operation of the latter to produce an open circuit in the input to amplifier 4 at the points 22. Similarly, operation of the codan circuit H at the west terminal, when its output is operative, causes the input of the amplifier-detector 4 associated with the transmitting circuit TW at the same terminal to be disabled at the point 23, as indicated.

However, if the frequencies of the carrier oscillators associated with the transmitting circuit at the two terminals of the system are different so that different radio frequencies are transmitted for the two directions of transmission, some or all of the above-described controls may be eliminated.

The system of the invention, as shown in the drawing, operates as follows:

When no speech signals are being received by the radio telephone system from the west tele phone line LW associated with the west radio terminal or from the east telephone line LE associated with the east radio terminal, the eon trol circuits at the two radio terminals are in the condition indicated in the figure, with both transmitting circuits TW and TE disabled in their outputs at the points 3 and 3"respectively, so that no carrier waves may be radiated to the other terminal, and with the receiving circuits RE and RW blocked in their outputs at the points 15 and I5. respectively. The normal blocking of the outputs of the receiving circuits prevents, when signals are not being transmitted, noise currents from being transmitted through the associated line to the telephone subscriber associated with each terminal, and being fed back through the transmitting circuit at the terminal to the switching apparatus for controlling the transmission of carrier from that terminal. In the absence of received carrier, the automatic gain control relays at each terminal are unoperated, and the gain of each receiving circuit RE, RW is set substantially at the value last established when carrier was removed, which value may be sufficient to make the noise currents present, if allowed to go through, of sufficient amplitude to annoy the subscriber associated with the receiving line LE or LW and to cause false operation of the carrier transmitting apparatus associated with the transmitting circuit at the same terminal.

Now, let it be supposed that speech waves are being received in the transmitting circuit TW of the west radio terminal from a subscriber associated with telephone line LW and speech waves are not being simultaneously received in the transmitting circuit TE of the east radio terminal from the telephone line LE associated The main portion of the speech waves in the circuit TV! will be impressed upon the input of the radio transmitter l and will modulate therein the carrier frequency from the associated carrier oscillator 2 in well- Simultaneously, a portion of the speech waves in the input of the radio transmitter I controls the device 4 to remove the dis-' ability at the point 3 in the output of the radio transmitter l so as to allow the side-band energy At' the east terminal of the system the radiated energy will be picked up by the receiving antenna RAE and will be selected and amplified by the radio frequency tuning and amplifying means 5 in the input of the receiving circuit REJThe resulting amplified energy is impressed on the first detector 6 in which it will be combined with the local carrier f2 supplied from the carrier oscillator 1 to produce waves of intermediate frequency. A combination-carrier component in the output of detector 6 will be selected by the amplifier-detector l2 and will control the operation of the codan and the gain increase portion of the automatic gain control circuit in the manner to be described later.

The remaining portion of the waves of intermediate frequency in the output of detector 6 will be amplified by the intermediate frequency amplifier 8, the gain of which is controlled by selected carrier waves in the input and output of the amplifier by the automatic gain control circuit in the manner to be described later, and then will enter the second detector 9 in which the speech signal waves will be demodulated in wellknown manner. The speech waves will then be amplified by the audio amplifier I.

A portion of the waves in the output of the first detector 6, i. e., a combination product of the carrier frequency f1 generated by the carrier oscillator 2 at the west terminal and the carrier frequency f2 generated by the local oscillator at the east terminal, will be selected in the common input of the codan circuit H and the forward-acting portion of the automatic gain control, connected across the output of the detector, and will be amplified and detected by amplifierdetector l2. The detected current will energize.

the windings of the codan relay I3 and the gain increase relay GI connected in parallel across the output of the device l2, causing their operation.

Relay It will operate to open the short-circuiting connection l5 across the receiving circuit RE in the output of audio amplifier l0, thereby allowing the speech signal waves in' the output thereof to be transmitted from the output of the circuit TE through the hybrid coil Hz to the line' LE over which they will be transmitted to the east subscriber. 'The operation of relay I3 also closes an energizing circuit for the winding of relay 2! from battery 24, causing the operation of the latter relay to open the input circuit of amplifier-detector 4' at the points 22 for the" purpose referred to previously.

The gain increase relay GI operates simultaneously with relay 13, to close its armature and contact applying a positive voltage from battery 25, through the charging resistance 26 and the normally closed armature and back contact of the gain increase disabler relay GID, to the regulating condenser 2? in the grid circuit of the intermediate frequency amplifier 8 causing the 30 current in the output of detector I6to push the gain of amplifier 8'down-in the following mangain of the latter amplifier to increase as the charge on? condenser .21' increases. The carrier diate frequency amplifier 8 due waves, as long as they. continue tof arrive, will holdthe relay ,GI; operated "and :for a desired hangover interval'after the reception of carrier Waves ceases. 7

The gain of the intermediate-frequency amplifier 8 will continue to be. increased by the chargin'g-of condenser 21 in response to the operation of relay GI until the'am'plified waves'in the out putof that amplifierreach the desired average levelwhen the detected current in the output of detector" lfi-connected across the output of amplifier 8 is suflicient to cause operation of the gain increase disabler relay GID in the output, circuit. of the detector.

Theiresultant operation of the relay' on) breaks 1 its. armature and back contact disabling the charging circuit for condenser'ZTfrom battery 25 through resistance 26, thereby-preventing any further increase irithe gain of the intermeoperation of the gain increase relayGI.

If the output level of amplifier 8 remains at the desired value, the gain. thereof will be sta-.

bilized at a value corresponding to the charge .f of the latter-to 'startto decrease as the 'if any,- "and GID are relatively on condenser 21 at the time relay GID operated.

however, the output level of "amplifier 8. increases about 1 decibel, the marginal gain decrease relay GD will be operated by the detected ner. 1

The relay GI operates to' connect the rather low discharge resistance'28 .in parallel with the regulating condenserl 'l in the gridcircuit of the intermediate amplifier tube 8 causing the gain condenser 21 discharges through resistance 28. 'The level offthe waves in the output'of the intermediate frequency amplifier -8 will decrease in accordance with the decrease in gain until thedetected cLu'-' rentin the'output of detector I6 is reduced-to the pointwhere the leases. 'The relay GD hangover in operation.

is designed to have little, The relays GD at which the gain decrease relay GD releases is not sufliciently low tocause'the release ,of the gain increase disablerrelay GID. When the gain decrease relay GD releases, the resistance 28is denser 21 until such time as the level of the waves disconnected from in parallel with the regulating condenser 21 and the gain of'theintermediate frequency amplifier is stabilized'at the value corresponding to the charge on regulating con in the output of amplifier' fi is reduced to the point where thegain increase disabler relay GID releases. The latter'relay preferably has no hangconnects The release of relay GID reover in operation.

- ing-condenser Z'Iso .that the operation of the gain 7 increase relay 1 rier will cause the gain of the intermediate frefquency'amplifier 8 to be increased in the manner previously described.

GI in response'to the received car- If at any timethe reception of carrier waves ceases with cessation in speech transmission at the transmitting terminal, each of the gain control relays which may be operated at the receiving terminal will release so that the gain control circuits are in the condition indicated in the drawing, and the. gain of amplifier 8 will remain substantially at the value last established corresponding to the charge on condenser 21 when to the continued gain decrease relay GD redesigned so that the level the charging resistance 26 to regulatcarrier was removed, although there' will bea' slightdrift to low gain leaks-oif the condenser. are arranged to have-a short hangover in operation, provided, for example, by,designing' themas the chargeslowly- V The relay GI and]? to be slow-releasing to the required degree, so

that their release will not result in signal clipping- The release of the codan relay 13 will cause'the I short-circuiting. connection 15 to be reapplied across the'output of amplifier l0. 7 By suitable selection of the constants ofthe circuit associated-with the gain increase'and decrease relays, i. 'e., the values of condenser 21, I resistances 26' and 28, and battery 25, the rates of gain increase anddecrease may be madefast enoughtto follow fading, but not :fast enough ;to follow thespeech envelope. j a The sensitivities of the codan circuit and that of V the, gain increaser: circuit operated therethrough are made exactly the same f so that the above-mentioned fundamental requirements forthe ideal codan circuit are QbtalhBdTbOth for it g and the gain increaser circuit of vthe automatic gain control, i. e., both will operate on the weak estusable carrier and will not operate on the;

strongest static that'can be over-ridden; It will be noted also that all of the gain regulation of beapparent to persons skil1ed-inthe,-art-; for; example,- as indicated above, control energy other than the carriersent out from 'the tra nse, mitting terminal may be used to control, the codan and automatic gain controlcircuits atthe' receiver; carrier'energy. may be used for adjust-1 rcuit s which Q ing receiver gain and control; energyof. another: frequency transmitted during'signal transmission 7 forcodanoperation-or vice versa; the codan Oil),-

r cult may be bridged at some point infront ofthe point of gain regulation otherthan. the output of the first detector, forexample, across the receiving antenna; at least a portion ofthe gain 5 regulation of the receiver may be made in the; audio amplifier; the gain increase: relay may be;

controlled from a different. amplifier-detector than used for controlling the codanrelay, and 5 it may be connected to the receiving circuit atia different point than they codan amplifier-detec tor; or the codanv function. and the gain increase employing two sets of contacts. What is claimed is:

u 1. A terminal station for a'two-way'signaling system including a two-way transmission mev dium, comprising a circuit for transmitting sigfunction may be accomplished by a single relay 7 1 nals to said medium anda circuit for receiving signals therefrom, transmission apparatus con;

nected. to the output of said receiving circuit, means normally disabling said receiving circuit near its output, automatic gain'regulatingmeans in said receiving circuit-switching; means connected to said receiving circuitin front of the,

'point of gain regulation therein, 'r'esponsive t'o impressed'control energy of distinctive frequency when signals are being received at the terminal station from said medium to remove the disability from. said receiving circuit, and means responsive to signal transmission in said transmitting circuit to disable said switching means, and to prevent change in the gain of said receiving circuit until said signal transmission ceases.

2. A two-way telephone system comprising two stations connected by a two-way transmission medium subject to static, each station comprising a transmitting circuit and a receiving circuit connected to said medium, a subscribers circuit coupled to the output of said receiving circuit and the input of said transmitting circuit, means normally disabling the output of said receiving circuit to prevent the transmission of static energy to said subscribers circuit and said transmitting circuit in the absence of received telephone signal energy, means responsive to the initiation of signals in said transmitting circuit for sending out over said medium to the other station said signals and control energy of distinctive frequency, means responsive to the control energy of distinctive frequencyreceived from the other station to automatically regulate the gain of said receiving circuit in accordance with the volume level of the received control energy, switching means connected to said receiving circuit in front of the point of gain regulation therein, selectively responsive to the received control energy to remove the disability from the output of said receiving circuit, means to maintain the gain of the receiving circuit at the last established value when the reception of said control energy ceases, and means responsive to the initiation of signals in said transmitting circuit to disable the switching means connected to said receiving circuit and to substantially prevent change in the gain of the receiving circuit during signal transmission from the station.

ALTON C. DICKIESON'. 

